Electrochemical double layer capacitor

ABSTRACT

An electrolytic double layer capacitor having low series resistance, high capacitance, and low inductance characterized by activated carbon plates of very high true surface to geometric volume ratio separated by a highly porous inert spacer as thin as 0.0005 inch, impregnated with highly concentrated electrolytes such as KOH or H2SO4.

United States Patent Hart et al.

[54] ELECTROCHEMICAL DOUBLE LAYE CAPACITOR [72] Inventors: Burt E. Hart,Red Hook; Richard M.

Peekema, Woodstock, both of NY.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: June 30, 1969 21 Appl. No; 837,600

[52] U.S. Cl ..3l7/230, 317/258 [51] Int. Cl. ..I-I0lg 9/04 [58] Fieldof Search ..317/230, 231, 233

[56] 1 References Cited UNITED STATES PATENTS 2,299,667 10/1942 Waterman..317/230 2,444,914 7/1948 Brennan ..317/230 [451 Mar. 28, 19722,580,399 1/1952 Brennan ..317/230 2,800,616 7/1957 ..317/23O 3,098,1827/1963 317/230 3,263,137 7/1966 317/230 3,288,641 11/1966 ..317/231 X3,443,997 5/1969 ...3l7/231 X 3,536,963 10/1970 Boas ..317/230 PrimaryExaminer-James D. Kallam AttorneyHanifln and Jancin and Melvyn D. Silver[5 7] ABSTRACT An electrolytic double layer capacitor having low seriesresistance, high capacitance, and low inductance characterized byactivated carbon plates of very high true surface to geometric volumeratio separated by a highly porous inert spacer as thin as 0.0005 inch,impregnated with highly concentrated electrolytes such as KOl-I or H 501 Claim, 3 Drawing Figures PATENTED AR2 I912 3.652.902

FIG. I

.1 E .08; T .06; w i .04 a

10 100 1K 10K 100K FREQUENCY (Hz) FIG. 2

FIG. 3

CAPACITANCE (MILLIFARADS) 100 H 10K 100K AGENT 1 ELECTROCHEMICAL DOUBLELAYER CAPACITOR FIELD OF THE INVENTION BACKGROUND OF THE INVENTIONCapacitors of various types are known in the art and used for differentpurposes. The purpose for which a capacitor is used will determine thetype of capacitor used. For example, one common distinction is todistinguish between polar capacitors and non-polar types. Non-polartypes can be used with AC or DC current, while polar types are used withDC current. The typical polar capacitor such as an aluminum electrolytictype, having characteristics of one-third farad capacitance, workingvoltage of 6 volts, l milliohms equivalent series resistance (ESR) andwith an inductance of approximately 100 nanohenries may averageapproximately 3 inches in diameter by 9 inches long in physical size.Thus, this type of capacitor has undesirable characteristics such aslarge bulk, a low capacity per unit volume, a series resistance that isoften very high, and an inductance that is very high even under optimumcircuit conditions. When aluminum electrolytic type non-polar capacitorsare used, the capacitance per unit volume is lower still than with thepolar type.

For many applications, such as for power supplies or load circuits, itis desirable that a capacitor have the properties of being non-polarwhile having a low series resistance, high capacitance, low inductance,and small bulk.

SUMMARY Thus, it is an object of this invention to provide a highcapacitance, low series resistance, low inductance capacitor of smallsize.

Another object is to allow such capacitors as above to be fabricatedinexpensively.

These and other objects are met by the capacitor structure of thisinvention. This capacitor comprises in one illustration, two blocks ofactivated electrode material such as activated carbon or highly porousgraphite material, exhibiting a very large true surface to geometricvolume, at least one square meter per cubic centimeter, separated by ahighly porous nonconductive material, of a thickness of approximatelymils, impregnated with a high conductivity electrolyte such as aqueouspercent H 80 This capacitance cell is connected via a non-porouselectronic conductor, inert to the electrolyte, to exterior metallicconductors and/or'to adjacent cells. Known potting compounds are used toprovide the structural support and to seal the individual cells againstleakage of electrolyte.

Utilizing the electrolytic double layer effect in this manner, a fourcell capacitor approximately 1% inches in diameter by one-fourth inch inthickness (overall) can provide, as an example, characteristics of 0.4farad capacitance with a working voltage of 4 volts, an ESR ofmilliohms, and DC leakage resistance greater than 10,000 ohms.

IN THE DRAWINGS GENERAL DESCRIPTION The electrical double layer effectis known in the art, and is discussed in such publications asElectrochemical Kinetics" by Vetter, 1965, Academic Press, Inc., pages73-103, and Fast Charge Molten Salt Batteries," Proc. of 21st AnnualPower Sources Conference, 1967, pages 42-45.

FIG. 1 shows a cross-section of a two cell, double layer capacitor inaccordance with this invention. The vertical scale has been greatlyenlarged to facilitate description of the device.

An activated carbon or highly porous graphite material 1, 5-40 mils inthickness in the preferred embodiment, is utilized as an electrode. Thismaterial exhibits a very large true surface to geometric volume. This isachieved by using activated carbon or highly porous graphite. Activatedcarbon is made by expanding the pores in the carbon to increase the truesurface area of the material. The term activated carbon is well known inthe art. For example, the book, Activated Carbon by J. W. Hassler,Chemical Publishing Co., New York, 1963, describes various methods ofmaking activated carbon. The electrode material, whether activatedcarbon or highly porous graphite or other material, must be highlyporous so as to expose a very great surface area. As a ratio of truesurface area to volume, a minimum of 1 square meter/cubic centimeter isnecessary for the desired improved capacity characteristic. Where DC orlow frequency (e.g. 60Hz.) service is of primary interest, a workingrange of at least 20-500 square meters/cc. is preferred. The porosity,of course, must be interconnecting to permit electrolyte access, and theabove ratio pertains to the working zone or layer of the electrodehaving such access. The electrode material must be inert to theelectrolyte used, and must be a good conductor of electricity. Themaximum thickness of this material is limited only by the allowableresistance and inductance that a designer can tolerate. The minimumthickness is dictated only by the fragility and working characteristicsof the material, in producing a practical device.

Thus, the electrode material must have a high surface area, be highlyporous, and be a conductive material. Such materials thus include highlyporous nickel electrodes as made by sintering, and platinum black. Thesematerials are given by way of illustration of the types of materials,and are not meant as limiting this invention solely to these materials.For clarity, these materials will all be defined as activated electrodematerial, having the above properties.

Thus, an activated electrode material is one having a highsurface/volume ratio at least 1 square meter per cubic centimeter, ofinterconnecting porosity, and be an electrically conductive material,while being essentially inert to the electrolyte used in conjunctionwith the electrode. Located between the activated electrode material 1is a highly porous non-conductor 2 impregnated with a high conductivityelectrolyte such as a 30 percent solution of H SO or a 25 percentsolution of KOH in water, in the preferred embodiment. Aqueouselectrolytes have, in general, higher conductivity but a lowerdecomposition potential than nonaqueous electrolytes. In most cases, anaqueous electrolyte is preferred because of the lower resulting seriesresistance. The desired voltage rating can be achieved by employing aplurality of cells in series.

The non-conductor material 2 functions both as an electrolyte holder andas an electrode separator and is one of the materials whichsignificantly influences the equivalent series resistance (ESR) of thecapacitor. This material is thin, preferably 0.5-10 mils, highly porous(greater than percent open spaces), and readily wet by the electrolyte.In essence, this material is as thin as possible while still preventingcontact between the adjacent activated electrodes. The greater thespacing, the greater is the electrolyte contribution to resistance.

While most capacitors made in accordance with this invention will havesuch a spacer, as a practical manufacturing expedient, it will be clearthat what is needed is a space between the electrodes to preventelectrical shorting, and not a spacer per se. Thus, if the electrodesare mounted in a spaced relation to each other by some mounting means,no spacer as such is needed.

Materials that are utilized include filter paper as is commonly found inmost laboratories, cellulose fiber papers, tissue papers, nylon mesh forbasic solutions, or porous plastics, polylmide, epoxy glass, glasscloths, or any material that is highly absorbent (readily wet), and thatwill maintain the separation between the electrode plates. The materialis as porous as possible so as to reduce resistance as much as possible.Thus, insulating meshes or mattings are also allowable.

Care should be taken that the non-conductor is sufficiently absorbent orwettable so as to avoid voids or bubbles being trapped therein, whichprevent the maximum amount of electrolyte being utilized and increasesresistance.

A non-porous electronic conductor element 3, which is inert and non-filmforming in the selected electrolyte, is adjacent the carbon blocks. Thismaterial prevents electrolytic conduction between cells, and alsoprevents galvanic corrosion of the metallic end pieces 4. A preferredchoice is gold, plated upon a metallic conductor such as copper. Anumber of choices for this material are available, including gold,platinum, base materials plated with gold or platinum, pyrolyticgraphite, vitreous carbon, or wax or resin impregnated graphite orcarbon. Elements 3 could be combined physically with electrode elements1 by utilizing a composite structure made from, for example, selectivelyimpregnated carbon. It is important that whichever material is chosen,it should neither react with the electrolyte, nor allow an oxide orother insulating film to form between the electronic conductor 3 and thecarbon blocks 1.

The metallic conductor 4, such as copper in the preferred embodiment, isused to provide both structural strength for and electrical contact tothe capacitor. If desired, the external form of these end pieces may bemodified to provide a more conventional connection such as a pigtailconnection, as with other types of capacitors. However, theconfiguration shown is designed to reduce the overall inductance of thepackage by maximizing the reluctance of the magnetic path around thedevice. For this reason, the overall package is disk shaped, that is,thin with respect to its lateral dimension, and circular in outline, andhas large, flat contact surfaces.

An insulating inert gasketing material 5, such as Teflon orpolyethylene, which is bonded to the electronic conductor, is utilizedin the preferred embodiment. The bonding agent is, of course, resistantto the electrolyte. This material is as thick as is necessary, dependingupon the thicknesses oflayers l and 2, and serves as a first line ofdefense against leakage of the electrolyte.

A potting or encapsulating material 6, such as an epoxy resin, providesa secondary means of holding the capacitor together. This material alsoserves as a second line of defense against leakage of the electrolytes.This material is at least moderately resistant to attack by theelectrolyte, and does not fracture easily.

The overall dimensions for a typical capacitor of this construction areapproximately one-fourth inch thick by A inch in diameter.

A preferred method of assembly of this capacitor is as follows. Prior toassembly of the capacitor, the electrode material is thoroughlyimpregnated with electrolyte. Vacuum impregnation is preferred, but isnot essential in all cases. It will, however, insure the maximum contactpossible between electrolyte and electrode, and is to be desired. Beforeimpregnation, the electrolyte may be deaerated by bubbling purifiednitrogen gas through it. The capacitor is then assembled in a nitrogenatmosphere.

The electrical connection between layers 3 and 4 in FIG. 1 is apotential trouble spot, as maximum conductivity is desired. If theelectronic conductor 3 is wax impregnated graphite, some conductiveadhesive may be needed to reduce the resistance between the conductor 3and the metallic conductor 4. Other approaches which obviate thisproblem are to use gold as the non-porous barrier, and electroplate itdirectly onto copper end pieces 4, as in the preferred embodiment. Ifpyrolytic graphite is used, this can also be deposited directly onto themetallic end piece. In any event, sound electrical connection must beassured between the layers 3 and 4.

The activated electrode material is the key material in providing thecapacitive properties of the double-layer capacitor. For example, mostconductors exhibit a double-layer capacitance of -50 microfarads/cm.when placed in an electrolytic solution, and carbon, for example, is noexception. When this property is combined with the huge surface area ofactivated carbon (which may be as great as 1,000 square meters per cc.),the result is as shown in examples below. The primary problem is gettingthe activated carbon in the desired physical form to build thecapacitor. Among the solutions to this problem are the four discussedbelow.

One may first fabricate a solid carbon material into the desired shape,and then activate it. Alternatively, one may take activated carbonpowder or granules and design the cell to utilize them directly. Or, onemay utilize commercially available sheets of filter media which containhigh percentages of activated carbon. Or, one may start with activatedcarbon powder and a suitable binder, and press into the desired shape.

EXAMPLE 1 Two blocks of carbon one-half inch thick, cut from a 2 inchdiameter carbon electrode, were separated by a single sheet of facialtissue paper soaked in a saturated solution of table salt. Thisarrangement gave a DC capacitance of 300,000 microfarads, an ESR of 15milliohms, and a leakage resistance of ohms at 1.1 volts.

EXAMPLE 2 A petroleum coke base carbon was fabricated into wafers 1 inchin diameter and 0.020 inch thick and then activated by heating in air at500 C. for several hours, until a surface to volume ratio of 50 sq.meters/cc. was generated. Utilizing aqueous 30 percent H 80, electrolyteand a microporous plastic separator, a two-cell capacitor was madehaving the following properties: DC capacitance 1.4 farads, ESR 58milliohms, working voltage 3 volts, and DC leakage resistance of 1,000ohms.

EXAMPLE 3 Activated carbon, as in Example 2 above, 0.010 inch thick,four cells in series, high frequency ESR (see FIG. 2)0.035 ohm, DCcapacitance 0.4 farad, inductance 0.2 nh., working voltage 4.0 volts, ACripple current, max. 4 amp (est.), leakage current at rated v. 0.5 ma.,and dimensions of 1.5 inch in diameter by 0.27 inch or 0.48 cu. in. TheAC performance characteristics are shown in FIGS. 2 and 3.

EXAMPLE 4 Activated carbon FC-13 Fuel Cell electrodes from Pure CarbonCo., St. Marys, Pennsylvania, 1.00 inch in diameter by 0.10 inch thick,having a surface to volume ratio of 400 sq. meters/co, 30 percent H 50,aqueous electrolyte, microporous plastic separator, DC capacitance 40farads, ESR 100 milliohms, and voltage 1 volt.

In summary then, in these capacitors, a thin layer of highly conductiveelectrolyte is sandwiched between two identical electrodes of anactivated electrode material having a very large ratio of true surfaceto geometric volume. The electrode area and separation are selected togive the desired equivalent series resistance,'and the true surface areaof the electrode will determine the value of capacitance. For example,smooth platinum exhibits approximately 20 microfaradslcm? double layercapacitance, while platinum black has been measured at approximately100,000 microliarads/cm The AC characteristics shown in FIGS. 2 and 3suggest that the participation of the more inaccessable areas of theelectrode surface is frequency dependent. Thus, although electrodematerials having very high surface/volume ratios (e.g. at least 20-500square meters/cc.) are preferred for DC or power frequency service,materials exhibiting higher conductivity at the cost of lower (but stillsubstantial) surface/volume ratios may be optimum for high frequencyservice.

The charge-voltage characteristic of the symmetrical electrochemicalcells are essentiallylinear up to the decomposition voltage of theelectrolyte, which will be about 1%volts, for preferred aqueouselectrolytes. To achieve higher working voltages, several cells are putin series, at some expense of capacitance and ESR. The preferreddisk-shaped construction of the individual cells and the overallpackage, and particularly the use of the axially compact and laterallyextensive disk 3 connection between cells, enables multiplication ofthe. number of cells while avoiding prohibitive inductance problems.Thus, the preferred electrolytic conductor is a solution of lowresistivity such as 30 percent H 80. or 25 percent 'KOH in water, asstable as possible to avoid thermal and electrochemical decomposition.Nonaqueous solvents could be selected to give a greater decompositionvoltage rating per cell, but such electrolytic conductors in generalhave a higher resitivity than their aqueous counterpart. Also, solidelectrolytes could be used to eliminate liquids entirely. For example,RbAgJ and KAg,,l as such electrolytes, exhibit high electrolytic typeconductivity and ionic mobility. Solid electrolyte use might reduce oreliminate gas problems. In each case, the

electrode is a chemically inert solid of large surface area such ascarbon or platinum, and is wet by or is in intimate association with theelectrolyte.

The separation between electrodes is maintained by a thin plasticgasket, or by a thin porous separator of paper or plastic which is notattacked by the electrolyte, or by some other suitable means. Thecompleted device may be composed of a number of cells in series formedby alternating layers of electrodes and electrolytes, with theelectrolyte in each cell essentially separated from that in all othercells. The completed device may be sealed with or without a safety vent,the latter being possible in some cases because of the small amount ofliquid present in each cell. As an additional feature, one could add athird electrode to each cell to provide for a catalytic recombination ofgases released during an over voltage condition, as is done in sealedsecondary batteries.

While specific examples have been shown, and specific materialsillustrated, others skilled in the art will realize that other materialsmay be substituted therefore while still maintaining the scope andcontent of this invention.

What is claimed is: 1. A low inductance electrolytic double layercapacitor comprising at least one cell comprising at least first andsecond thin,

disc-form electrode structures,

spacing means for maintaining said electrode structures in spacedrelation to each other,

each of said electrode structures comprising an electrode portion of thesame activated material presenting on one side a porous generally planarspace contiguously to the space between said electrode structures,

each said structure further comprising a planar electrolyte imperviousconductor portion substantially coextensive with and in electroniccommunication with said activated portion at the other side thereof,

said activated portions and said space being saturated with a dryionically high conductive solid electrolyte, said electrode structuresand said means for maintaining the same in spaced relation being inertto said electrolyte,

said electrolyte being a solid ionic conductor consisting of silveriodide containing an alkaline metal, said electrolyte being chosen fromthe group consisting of RbAg l and KAg4l5,

and an axially short outer package comprising conductive, disc-shapedend terminals parallel to and in electrical communication with saidconductor portions and substantially coextensive therewith,

said cell being connected in series between said terminals by saidelectrolyte impervious portions, providing non-film forming isolation ofthe electrolyte to the cell and electrical communication to saidterminals, said impervious conductor portions comprising portions ofsaid electrode structures which are selectively sealed so as to bedevoid of said elec trol yte

1. A low inductance electrolytic double layer capacitor comprising atleast one cell comprising at least first and second thin, disc-formelectrode structures, spacing means for maintaining said electrodestructures in spaced relation to each other, each of said electrodestructures comprising an electrode portion of the same activatedmaterial presenting on one side a porous generally planar spacecontiguously to the space between said electrode structures, each saidstructure further comprising a planar electrolyte impervious conductorportion substantially coextensive with and in electronic communicationwith said activated portion at the other side thereof, said activatedportions and said space being saturated with a dry ionically highconductive solid electrolyte, said electrode structures and said meansfor maintaining the same in spaced relation being inert to saidelectrolyte, said electrolyte being a solid ionic conductor consistingof silver iodide containing an alkaline metal, said electrolyte beingchosen from the group consisting of RbAg4I5 and KAg4I5, and an axiallyshort outer package comprising conductive, discshaped end terminalsparallel to and in electrical communication with said conductor portionsand substantially coextensive therewith, said cell being connected inseries between said terminals by said electrolyte impervious portions,providing non-film forming isolation of the electrolyte to the cell andelectrical communication to said terminals, said impervious conductorportions comprising portions of said electrode structures which areselectively sealed so as to be devoid of said electrolyte.